Record-breaking heat is expected in the Southland this weekend, according to the National Weather Service.

“Temperatures well above normal ... will be prevalent by Friday,” said an NWS statement. “By the weekend, record-breaking heat is forecast ...”

NWS forecasters said Saturday and Sunday highs could be 20 degrees above normal. They attributed the expected hot spell to an unseasonably hot air mass that will develop this weekend as a strong ridge of high pressure aloft builds in the region.

“By the weekend, record-breaking heat is forecast, when the center of the high pressure ridge moves over the region,” the statement said.

Daytime highs Saturday and Sunday will range from the 80s to lower 90s at the coast and from the mid 80s to mid 90s in the valleys and foothills, the statement said. NWS forecasters urged residents and visitors to avoid heat stress, including by scheduling outdoor activities in the morning or evening to avoid the day’s strongest heat, wearing light clothing if engaging in strenuous activities, and staying hydrated. Also forcefully recommended: “Never, ever leave children, pets or the elderly alone in the car.”

The weather service expressed concern for people who’ll take part in Sunday’s L.A. Marathon, saying they need to be aware of the potential for serious heat-related problems. Also appearing concerned are the marathon’s organizers. The entire field will start at Dodger Stadium at 6:55 a.m., 30 minutes earlier than originally scheduled in an effort to beat the heat.

Other steps will be taken to better ensure runner safety. The race will have wave starts to ensure proper runner spacing and enable more efficient replenishment of supplies at aid stations. Additional water and Gatorade Endurance Formula will be provided at all 24 aid stations.

“Cooling buses” will be stationed along the course and at the finish line; misting stations, cold towels and ice will be provided.

“While we are anticipating warmer temperatures for Sunday’s race, we are confident in our level of preparedness and the resources we have available to support a positive race day experience for all participants,” Dr. Glenn Ault, the race’s medical director, said this week.

Sunday’s high in metropolitan Los Angeles, an area that takes in the marathon course, is forecast to be 91.